Vacuum ultraviolet pulsed field ionization study of ND_3:Accurate thermochemistry for the ND_2-ND_2~+ and ND_3-ND_3~+ system

摘要

The dissociation of energy-selected ND_3~+ to form ND_2~+ D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method.The breakdown curves for ND_3~+ and ND_2~+ give a value of 15.891+-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND_2~+ from ND_3.We have also measured the PFI-PE vibrational bands for ND_3~+(X;v_2~+=0,1,2,and 3),revealing partially resolved rotational structures.The simulation of these bands yields precise ionization energies (IEs) for ND_3~+ X(0,V_2~+=0-3,0,0)<-ND_3x(0,0,0,0).Using the 0 K AE (ND_2~+) and IE(ND_3)=10.200+-0.001 eV determined in the present study,together with the known 0 K bond dissociation energy for ND_3 [D_0(D-ND_2)=4.7126+-0.0025 eV],we have determined the D_0(ND_2~+-D),IE(ND_2),and 0 K heat of formation for ND_2~+ to be 5.691+-0.001 eV,11.1784+-0.0025 eV,and 1261.82+-0.4 kJ/mol,respectively.The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND_2~+),indicating that the dissociation of excited ND_3~+ at energies slightly above the dissociation threshold is prompt,occurring in the time scale <=10~(-7) s,as observed for the NH_3 system.The available energetic data for the NH_2-NH_2~+ and NH_3-NH_3~+ system are found to be in excellent accord with those for the ND_2-ND_2~+ and ND_3-ND_3~+ system after taking into account the zero-point vibrational energy corrections.This finding indicates that the thermochemical data for these two systems are reliable with well-founded error limits.